1. Field of the Invention
The present invention relates to an optical pickup including an aberration correction mechanism for correcting spherical aberration.
2. Description of Related Art
In an optical disc device, recording/playback of information is performed by irradiating laser light to an optical disc which is a recording medium by using an optical pickup. On a recording surface of the optical disc, grooves called as tracks are formed, and the optical pickup records or plays back information by allowing laser light to converge and irradiating the light to the tracks as a laser spot or by receiving laser light reflected by the tracks.
Since the width of the track is different in CD (Compact Disc) media and DVD (Digital Versatile Disc) media which have larger recording information capacity than the CD media, a diameter of the laser spot at the time of recording and playback is also different. The laser spot has a circular shape, having characteristics that a diameter thereof is in proportion to a wavelength of laser light and is in inverse proportion to a numerical aperture (NA) of an objective lens.
Infrared laser (wavelength: approximately 780 nm) is used for recording/playback of CD media. In DVD media having larger storage capacity, that is, having higher storage density than CD media, the track width is narrow, therefore, red laser (wavelength: approximately 650 nm) having shorter wavelength than infrared laser is used.
In recent years, BD (Blu-Ray Disc) media having larger recording information capacity than DVD media have appeared on the market. Since BD media has higher recording density and a narrower track width as compared with DVD media, a diameter of the laser spot is required to be small. Accordingly, laser light of blue-violet laser (wavelength: approximately 405 nm) having a shorter wavelength than the laser light for DVD is used for the optical pickup performing recording/playback of BD media. Concerning NA of the objective lens, NA of the objective lens for DVD is approximately 0.6, whereas the objective lens having higher NA, namely, approximately 0.85 is used for BD.
When laser light transmits through the objective lens and when transmitting through a transparent layer of a medium, spherical aberration occurs. The spherical aberration increases in proportion to biquadratic of NA of the objective lens and the thickness dimensions of the transparent layer of the optical disc. As the spherical aberration increases, the spot diameter of the laser spot also increases, as a result, it is difficult to read information recorded in the recording surface. In addition, as the spherical aberration increases, light intensity at the center decreases, as a result, it is difficult to record. A cover layer of the optical disc has a prescribed thickness as a standard, however, the thickness of the cover layer through which the laser light transmits is not even due to errors during manufacture or side-runout, and there is a case that the spherical aberration amount varies according to variation of the thickness of the cover layer and it is difficult to record or read stably.
For example, when comparing the objective lens for DVD (NA=0.6) with the objective lens for BD (NA=0.85), the spherical aberration amount when using objective lens for BD will be quadrupled as compared with the case using the objective lens for DVD. The DVD media only a little affected by the spherical aberration and can perform recording/playback of information, however, the above problem is liable to occur in BD media, which causes trouble in recording/playback of information in many cases. Even when using DVD media, in the case of performing recording/playback of DVD media which have upper-and-lower two layers of recording surfaces, the thickness of the transparent layer through which laser light transmits differs, therefore, recording/playback failures due to the spherical aberration are liable to occur as compared with one-layer DVD media.
Accordingly, a spherical aberration correction mechanism correcting spherical aberration by allowing plural aberration correction lenses to come close to or draw away from each other is used, which is arranged at a previous stage of the objective lens. The spherical aberration correction mechanism includes a movable portion holding at least one of the plural aberration correction lenses, which is slid in an optical axis direction of laser light, a feed screw having spiral grooves formed at an outer periphery, which is arranged in parallel to the optical axis of laser light and an engaging portion connected to the movable portion, which is engaged with the feed screw.
In the spherical aberration correction mechanism, when the feed screw is rotated by an electrical motor, the engaging portion moves straight in a direction along the center axis of the feed screw and the aberration correction lens held by the movable portion is slid in the optical axis direction of laser light to adjust the distance between plural aberration correction lenses properly. The laser light transmits through the plural aberration correction lenses, thereby correcting spherical aberration occurring when the light transmits through the objective lens and when the light transmits through the transparent layer of the optical disc. According to this, the optical pickup can make laser light converge accurately and can focus the accurate laser spot on the tracks formed on the recording surface of the media, as a result, recording/playback of information can be accurately executed.
As a method of driving the aberration correction lens, for example, a method disclosed in JP-A-2003-45068 is proposed. In the optical pickup, a lens holder to which a knife edge (engaging portion) is attached is arranged opposite to the knife edge, sandwiching the center axis of a feed screw, and the knife edge attached to the lens holders is engaged with the feed screw. Then, force is added to the lens holder in the axial direction of the feed screw to suppress the formation of a gap between the knife edge and the feed screw.
According to the above, occurrence of backlash can be suppressed, in which timings of rotation of the feed screw and operation of the knife edge do not match each other when the knife edge is pushed by the rotation of the feed screw. Accordingly, when the feed screw is rotated, the knife edge is also slid without delay, which enables the lens holder to move to a correct position at short time.
Additionally, in the optical pickups disclosed in JP-A-2003-45068 and JP-A-2007-18680, the knife edge and the feed screw are engaged with each other without a gap by pushing the knife edge to the feed screw in the diameter direction of the feed screw using a blade spring.
Also in this case, occurrence of backlash can be suppressed, in which timings of rotation of the feed screw and operation of the knife edge do not match each other when the knife edge is pushed by the rotation of the feed screw. Accordingly, when the feed screw is rotated, the knife edge is also slid without delay, which enables the lens holder to move to a correct position at short time.
Furthermore, there exist a technique in which the knife edge itself is formed to be a shape which can demonstrate elastic force, and the knife edge is pushed to the feed screw.
However, in the case that the lens holder has a shape of sandwiching the feed holder, when the lens holder is fixed due to unusual situations such as the movable range is exceeded or a foreign substance is caught, large force acts on both members due to engagement of the knife edge and the feed screw, which may deform and damage the knife edge or the feed screw. In addition, when power continues to be supplied to a motor for driving the feed screw in a state in which the engagement between the knife edge and the feed screw is not disengaged, the load of the motor is increased and the motor is heated, which may fail the motor itself or deform or damage peripheral members.
The blade spring adjusts elastic force by the deflection amount, and when the length of the blade spring is not sufficient, large elastic force is brought out by the small deflection amount. In the aberration correction mechanism, there is not sufficient space in a place where the blade spring is installed and the knife edge is pushed to the feed screw with a short blade spring in many cases. In such cases, the elastic force of the blade spring becomes too large, as a result, friction between the knife edge and the feed screw increases. Accordingly, abrasion of the knife edge and (or) the feed screw occurs in an early stage. In addition, the load of the motor increases or step-out of the motor occurs, therefore, it is necessary to increase the performance of the motor or to use the motor in a low-frequency range, which may lead to the increase of costs or accuracy deterioration of aberration correction.
When the knife edge is formed so that the knife edge itself demonstrates elastic force, the shape becomes complicated, which may decrease reliability such as the strength and the shape of the knife edge itself. In order to manufacture the optical pickup so as to have sufficient reliability, the high accuracy is required for the process, which leads to the increase of costs for manufacture.